(a) Field of the Invention
The present invention relates to a shift control method for an automatic transmission of a vehicle especially when the vehicle runs up a slope while towing a trailer.
(b) Description of the Related Art
Recently, the number of vehicles towing trailers with various leisure equipment has tended to increase as leisure activities have diversified. In this case of a vehicle towing a trailer, a large traction force are required.
When the vehicle towing the trailer is a manual transmission equipped vehicle, it is possible for a driver to appropriately perform shift operation while considering the present load condition of the vehicle.
However, in the case of a vehicle equipped with an automatic transmission, the shift operation is automatically performed according to the vehicle""s conditions such as running speed, throttle opening level, and so on by a transmission control unit (TCU) controlling various valves of a hydraulic pressure system cooperating with a gear train, such that intentional shift operation by the driver is limited.
Accordingly, it is required to adapt a separate shift control logic for when the vehicle tows a trailer, but this kind of logic has not been adapted to automatic transmission control until now such that the shift operation has been performed in the same shift pattern as a normal state even when towing a trailer.
If the shift control is performed with the normal shift pattern, especially when the vehicle runs up a slope with the trailer, generally second speed range is selected. However, a vehicle running in the second speed range experiences a lack of driving torque and an extreme slippage of the torque converter that causes the oil temperature to heighten.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a shift control method of an automatic transmission for a vehicle especially when the vehicle runs up a slope while towing a trailer so as to obtain enough driving torque, reduce slippage of the torque converter, and prevent the oil temperature from being heightened.
To achieve the above object, the shift control method for an automatic transmission of the present invention comprises the steps of determining if present shift lever position indicates a driving range 2, calculating weight-slope resistance if the shift lever position indicates the driving range 2, determining if the weight-slope resistance is greater than a threshold weight-slope resistance, measuring time duration during which the weight-slope resistance is maintained over the threshold value if the weight-slope resistance is greater than the threshold weight-slope resistance, determining if the time duration is longer than a threshold time, converting a present normal shift pattern into a trailer-towing shift pattern if the time duration is longer than a threshold time, and returning the trailer-towing shift pattern to the normal shift pattern if the weight-slope resistance becomes equal to or less than the threshold weight-slope resistance and this condition is maintained over a second threshold time.